1. Field of the Invention
The present invention relates to a transmitter and receiver of digital video signal capable of transmitting at a low bit rate.
2. Description of the Related Art
Digital video data of a dynamic image contains a very large amount of information and a transmission bit rate becomes very high if it is transmitted as it is. Due to that, conventionally data is compressed taking an advantage that generally an image is information of time and space when it is transmitted using a low rate transmission medium whose transmission bit rate is limited in a same manner as digital video data is recorded or reproduced using a magnetic tape or magneto-optic disk.
The compression and transmission of digital video signal have been conventionally carried out for example by taking a difference between frames of digital video data and by data compressing the difference using DCT, (discrete cosign transform). Further, considering that there are less motions in video, data is transmitted culling out its amount in a TV conference by transmitting data of only single side field, not transmitting,full frames.
Although the image may become fine by victimizing time data and by assuring space data by reducing a number of fields to be transmitted, not transmitting the single side fields, the motion may become awkward.
However, because a quality of the dynamic image is lost if the motion is awkward, a method of transmitting a full frame by compressing image data per one frame as much as possible to be able to reproduce a full motion as a dynamic image is being considered.
As described above, the conventional method for transmitting digital video signal is devised so that the transmission rate is lowered by compressing video data in the time direction or spatial direction.
However, if the transmission bit rate is to be lowered just by compressing data, i.e. by compressing in the time direction for example, a motion of the dynamic image becomes awkward and an unnatural video is brought about. Further, if data is compressed in the spatial direction, the spatial resolution is degraded that much, degrading the reproduced video.
That is, because the conventional method lowers the bit rate just by compressing data, it victimizes either or both data in the time direction and spatial direction, disallowing to obtain a satisfactory reproduced video.
Accordingly, it is an object of the present invention to provide a transmitter and receiver for digital video signal devised to be able to obtain a beautiful reproduced video as much as possible and in the same time, to render the motion smooth.
In order to solve the aforementioned problems, a transmitter for transmitting digital video signal of the present invention comprises (the reference numerals designated in the preferred embodiments described later are given here);
separating means 22 for separating an input digital video signal into background plane data representing a still image of a background image composing a screen image thereof and one or more motion plane data representing a still image of each moving object moving on the background image;
memory means 23BG and 23A1 through 23An for individually storing the separated background plane data and each motion plane data;
change information detecting means 22 for detecting information on changes of the still image stored as the motion plane data based on the input digital video signal and output of the memory means;
coding means 26 for compressing and coding an output of the change information detecting means; and
transmitting means for transmitting the still image data of the plurality of plane data in the memory means and the change information from the coding means.
According to the present invention constructed as described above, one scene composed of specific video contents for example may be recognized as a background image which can be considered as a stationary still image and one or more moving objects having different-motions.
That is, a 2-D image of one frame of a dynamic image is understood as a superposition of 2-D planes composed of each still image representative of the background and the plurality of moving objects. For the moving objects, information on changes of the moving object such as a moving direction, moving distance, deformation and the like are extracted. Then the change information on the motion plane is transmitted together with the background image plane and motion planes as dynamic image data.
At the receiver side, the dynamic image is reproduced by superpositioning the moving objects in the motion planes on the background image in the background plane corresponding to the change information.
In this case, each of the background plane and motion planes of the moving objects is one still image and data on the still image just needs to be transmitted at the very beginning of one scene. Because just each one still image needs to be transmitted for one scene of more than several seconds, a large amount of data may be transmitted even in a low transmission rate, thereby enhancing the image quality. Further, because the change information needs to be several bits at most, it can be transmitted on real-time with enough margin even in a remaining time after transmitting the still image data in one scene.
The above and other advantages of the present invention will become more apparent in the following description and the accompanying drawings in which like reference numerals refer to like parts.